1. Field of the Invention
The present invention relates to an ultrasonic rotational speed sensor, such as that used in an automobile to detect the rotational speed of wheels of the automobile.
2. Description of the Background Art
An example of a conventional ultrasonic rotational speed sensor is shown in FIG. 1. This conventional ultrasonic rotational speed sensor comprises: a disk shaped rotational member 10 having a plurality of cut out spaces like on a gear 10a formed at constant intervals along its circumference which coincides with its direction of rotation, which is only partially depicted in FIG. 1; and a transmitter microphone 11 for transmitting ultrasonic signals 13 and a receiver microphone 12 for receiving the ultrasonic signals transmitted from the transmitter microphone 11, which are arranged face to face with the rotational member 10 located therebetween. Although not depicted in FIG. 1, the transmitter microphone 11 is equipped with a signal generator for generating driving signals to be fed to the transmitter microphone 11 through a driver circuit, while the receiver microphone 12 is equipped with a pre-amplifier, a waveform rectifier, and a counter, which together form a rotational speed detection means.
In this conventional ultrasonic rotational speed sensor, the transmitter microphone 11 transmits the ultrasonic signals 13 toward the receiver microphone 12 while the rotational member 10 rotates, and only those ultrasonic signals 13 which pass through the gear cut-like portions 10a of the rotational member 10 reach the receiving microphone 12, so that the rotational speed of the rotational member 10 can be determined by detecting intervals of the ultrasonic signals 13 received by the receiving microphone 12.
Here, as shown in FIG. 2(A), the driving signals fed to the transmitter microphone 11 has a driving period which is sufficiently longer than a period of the rotational speed to be detected, where an appropriate length of non-driving period is provided between the driving periods in order to prevent an overheating of the transmitter microphone 11. For instance, in the case where the rotational speed to be detected is in a range of 60 r.p.m. to 3000 r.p.m. (equivalent to 20 msec. to 1000 msec. in terms of periods), the driving period of 5 sec. may be employed along with 1 sec. of the non-driving period, since the maximum period of the rotation is equal to 1 sec.
Then, as shown in FIG. 2(B), with a slight lag behind the driving signals, the ultrasonic signals 13 are transmitted from the transmitter microphone 11 toward the rotational member 10 as transmitted signals. The transmitted ultrasonic signals reach the receiver microphone 12 only when one of the gear cut-like portions 10a of the rotational member 10 is located on a path for the ultrasonic signals 13 formed between the transmitter microphone 11 and the receiving microphone 12. Thus, the receiving microphone 12 receives the ultrasonic signals 13 in a form of received signals shown in FIG. 2(C) which have the period synchronized with the period of the rotation of the rotational member 10. These received signals are then rectified by the waveform rectifier of the rotational speed detection means connected to the receiving microphone 12, and the rotational speed of the rotational member 10 is obtained by determining the period t of the rectified signals, as shown in FIG. 2(D).
Now, such a conventional ultrasonic rotational speed sensor has been associated with a problem of difficulty in positioning various electronic devices incorporated as elements of the rotational speed detection means connected to the receiving microphone, because the transmitter microphone and the receiving microphone are to be arranged face to face with the rotational member placed therebetween. For example, in a case of detecting the rotational speed of the wheels of an automobile, if the various electronic devices are to be arranged on a fixed substrate located on the same side as the receiving microphone, it has been necessary to take a complicated configuration such as that using slip rings for the sake of power supply to the devices and signal transmission from the devices.